Apparatus for drying heat sensitive products



H. RAVET 3,010,216

APPARATUS FOR DRYING HEAT SENSITIVE PRODUCTS Nov. 28, 1961 Filed Nov.15, 1957 INVENTOR. Huh 2' H d United States Patent Ofiice 3,010,216Patented Nov. 28, 1961 3,010,216 APPARATUS FOR DRYING HEAT SENSITIVEPRODUCTS Heinz Ravet, Koln, Germany, assignor to National ResearchCorporation, Cambridge, Mass, a corporation of Massachusetts Filed Nov.15, 1957, Ser. No. 696,697 Claims priority, application Germany Nov. 17,1956 3 Claims. (Cl. 34-92) Devices are already known in which the dryingof sensitive substances, for example synthetic fiber raw materials andorganic fiber materials, is carried out in a super-heated steamatmosphere. The presently disclosed invention stems from the problem ofattaining a specially effective and economical construction of such adrying unit. The unit is made in such a way that first a crude drying iscarried out in a current of super-heated steam which is supplemented bya more complete drying under high vacuum.

The present invention is directed to an improved apparatus and processwherein a dehydration unit for the crude and fine drying of sensitivesubstances employs a drying medium, which is circulated in a continuousmanner and serves as a heat carrier which acts on the material to bedried. The circulation of the drying medium is by means of a rotarycompressor (blower) which is preferably of the Roots type. This isconnected in a continuous circulatory system between a condenser and asteam heater.

A preferred form of such a unit employs the rotary compressor firstduring the so-called crude drying for circulating the chieflyvapor-phase (e.g. steam) drying medium. The rotary compressor is thenconnected, after the shift to fine drying, as the high vacuum stage of amultistage pump set-up.

The unit can be further developed in a favorable way so that a heateddrying vessel is provided for receiving the material to be dried, thisvessel being connected to the rotary compressor through a continuouscircuit with a condenser. A super-heater for steam is preferably locatedin the continuous system between the rotary compressor and dryingvessel.

A multistage gas ballast pump is preferably provided for thepre-evacuating of the continuous system and as the fore pump for thehigh vacuum pump in the fine drying of the material. This pump can beconnected to the continuous system between the rotary compressor and thesteam superheater.

A unit so constructed in contrast to arrangements with isolated steamgenerators, perhaps steam boilers, has important advantages. Thenecessary steam comes from the very material to be dried. For this it isnecessary that the material at the start have a satisfactory amount ofmoisture. This condition, however, always exists when using dehydrationin a current of super-heated steam, since this process is mainlyconsidered for crude drying.

In the drawing there is a diagram of an example of the invention. Ofcourse, other arrangements can be ad vantageously made by application ofthe idea in the invention.

In a drying chamber 1, which is provided with a heater 2 preferably inthe form of a super-heated steam coil, there is located the material 4to be dried on a perforated support 3. From the drying chamber 1 acontinuous conduit '5 leads next to a steam condenser 6 which has on theinside corresponding cooling surfaces in the form of a condenser coil 7.At the steam condenser there is attached, in a known manner, a trap 14.

The continuous conduit leads further to a Roots type rotary compressor 8and from there into a steam superheater 9 where a heating medium in aheating coil 10 can be introduced. From the steam heater 9 thecontinuous conduit 5 can finally be brought again to the drying chamber1 from which it started. Thus, there exists a closed circuit for thegaseous drying medium. Between the rotary compressor 8 and the steamsuperheater 9, there is inserted into the continuous conduit 5 a furthersupply line '11 which, by means of valve 12, is connected with a pumpfor removal of the vapor-containing mix ture, preferably a gas ballastpump 13 which is preferably two-stage. Between the Roots compressor 8and the steam superheater there is a valve 15 in the continuous conduit5.

The operation of the unit is as follows: First the material to be dried4 is placed on the perforated support 3 and introduced into the dryingchamber 1. The heating device 2 on the drying chamber 1 as well as thecondenser coils 7 in the condenser 6 and the heating coils 10 in thesteam superheater 9 are still not yet in operation. The rotarycompressor 8 likewise is not operating. The valves 15 and 12 are opened,the gas ballast pump 13 is placed in operation and there is apre-evacuation of the entire system (i.e., drying chamber 1, continuousconduit 5, condenser 6, rotary compressor 8, and steam superheater 9).example a few mm. Hg Abs. are obtained, the valve 12 is closed and thetwo-stage gas ballast pump 13 stopped. Now the material to be dried,which lies on the perforated support 3, is heated by bringing steam tothe heater 2 so that an adequate amount of steam is formed. At the sametime, the condenser 6 and the steam superheater 9 are put intooperation, while through the cooling coils 7 of the condenser 6 iscirculated a coolant of appropriate temperature, preferably cold water.Likewise, a suitable heating medium is circulating through the heatingcoil 10 of the steam superheater. Now, too, the rotary compressor 8 isswitched on and there begins a circulation of the gaseous drying mediumwhich originates in the charging material. The saturated steam leavesthe drying chamber 1 and is deposited in part on the condenser 6. Thesteam leaving condenser 6 now travels through the continuous circuitinto the rotary compressor 8, which causes the circulation to occur.From there the steam is circulated into the steam superheater 9 and issuperheated by the heating coil 10. The super-heated steam enters anewinto the drying chamber and can again remove moisture from the materialbeing dried. The circulating process described is continued until thedesired crude drying is finished.

At this point the crude drying is interrupted and the unit now switchedto fine drying. This is accomplished by shutting off trap '14 andclosing valve 15, While at the same time opening valve 12. The rotarycompressor 8, which has been up to this point causing the circulation ofthe current of steam, now serves as the high vacuum pump to which isconnected the two-stage gas ballast pump 13 through feed line 11.Thereby there occurs evacuation of the inner chamber of the drying area1 to lower pressures so that the existing steam is now removed from thesystem. Thus, under further pumping with increasing vacuum a fine dryingoccurs. To this end an additional heating of the material being driedcan be accomplished for which it is possible to utilize in the finedrying the heat generated during the crude drying so that furtherheating need not be used.

Especially advantageous is the use in the present arrangement of therotary compressor which during the crude drying process causes thecirculation of the steam current and after the switch to fine dryingserves as high vacuum pump. When the gas ballast pump inserted istwo-stage very low final pressures in the micron range Aftercorrespondingly lower pressure values, for- (of the order of magnitudeof 10- mm. Hg Abs.) can be attained.

The heating of the drying chamber 1 in the crude drying process and thecooling action of the condenser 6 must be so related to each other inthe operation of the unit during the crude drying that the desired steampartial pressure (e.g. 1 mm. Hg Abs.) exists in the system. During thesubsequent .fine drying the condenser 6 and steam superheater 9 can bedisconnected by interruption of the corresponding operating mechanismfor the cooling coils 7 and for the heating coils 10.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description, and shownin the accompanying drawing, shall be interpreted as illustrative andnot in a limiting sense.

What is claimed is:

1. A vacuum drying apparatus for drying heat-sensitive products, saiddrying apparatus being of the type wherein continuously recirculatingsteam at subatrnospheric pressure is employed for heating the product tobe dried, said apparatus comprising means defining a drying chamber forholding the product to be dried, a condenser, a blower for circulatingsteam, a steam superheater, a first conduit means for connecting thedrying chamber, the condenser, the blower and the superheater in seriesso that the blower can circulate superheated steam across the product tobe dried, through the condenser, through the blower and back through thesuperheater, a first valve means in said first conduit means between theblower and drying chamber on the exit side of the blower, a secondconduit means connected to the first conduit means between the blowerand the first valve means, a backing vacuum pump connected to saidsecond conduit means and a second valve means for isolating said backingpump from said first conduit means, whereby said first and second valveswhen open permit pie-evacuation of said drying apparatus by the backingvacuum pump and permit connection of said blower and backing vacuum pumpin series to provide a pressure on the order of 10 mm. Hg Abs. at theinlet of the blower and a correspondingly low pressure over the productto be dried when said first valve is closed and said second valve isopen.

2. The drying unit of claim 1 wherein the compressor is a positivedisplacement blower of the Roots type.

3. The drying unit of claim 1 wherein the backing vacuum pump is a gasballast pump.

References Citedin the file of this patent UNITED STATES PATENTS Re.24,260 Thies Dec. 25, 1956 1,324,072 Shorman Dec. 9, 1919 1,799,248Reinhardt Apr. 7, 1931 2,080,179 Merriam et a1 May 11, 1937 2,285,331Doyle June 2, 1942

